Tofua Arc at 21.1°S Volcano — Tonga

A large submarine volcano rises to within 65 m of the ocean surface west of Tongatapu Island. The volcano was informally named Volcano 1 by scientists on the bathymetric survey that mapped it in 2003. The summit of the andesitic-to-dacitic seamount is cut by a large 4.5 x 7 km caldera, with two young scoria cones forming the high point. A chain of explosion craters up to 100 m deep cut the flank of one of the scoria cones, and thick deposits of ash and scoria cover the caldera floor nearby. The lack of organic sediments between volcaniclastic deposits exposed in one portion of the caldera wall suggest it was constructed within the past 200 years. Diffuse low-temperature hydrothermal vents and vigorous gas discharge occurs near the explosion craters.